Rust-inhibitive abrasive for abrasive blasting



United States Patent 3,498,768 RUST-INHIBITIVE ABRASIVE FOR ABRASIVE BLASTING Isidore Geld, Flushing, Leo Deutsch, Brooklyn, and

Frank DOria, Bayside, N.Y., assignors to the Umted States of America as represented by the Secretary of the Navy No Drawing. Filed Oct. 18, 1967, Ser. No. 676,684

Int. Cl. B24c 11/00 U.S. Cl. 51-295 7 Claims ABSTRACT OF THE DISCLOSURE Method for preparing a rust-inhibitive abrasive material for cleaning metal surfaces by abrasive blasting. An aliphatic acid, such as stearic acid, is mixed with the abrasive material, such as black slag particles, at room temperature. The stearic acid forms about of the mixture. The mixture is heated to a temperature which melts the aliphatic acid and then cooled to room temperature, the mixture being continuously stirred while cooling. The process produces dry, granular abrasive particles which are coated with a film of fatty acid.

The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

This invention relates to a rust-inhibitive abrasive material and to a method for preparing this material.

Air blasting with abrasive material is a rapid way of preparing large surfaces of metal for painting. A difliculty frequently encountered in naval environments is the quick rusting of the freshly blasted metal due to high atmospheric humidity. To minimize rusting, a temporary primer should be applied within two hours after blasting. However, it is often impractical to apply coatings within this time limitation and coatings are expensive from the standpoint of the labor, material and time involved.

Wet abrasive blasting with a rust inhibitor in the slurry is often used but the protection it offers can be insufficient and it has been found that the wet process frequently results in an acceleration of rusting rather than an inhibition.

Dry blasting processes have been developed which involve metal shot, such as zinc or lead shot. Blasting with such metal shot leaves a metallic deposit on the cleaned surface, the deposit preventing combination of oxygen with the underlying metal. However, the use of metal shot is considerably more expensive than the use of an abrasive material consisting of particles of black slag, which is an abrasive blasting material favored when considerable amounts of blasting are to be done; also the protection offered by zinc, for example, is not completely satisfactory.

An object of this invention is therefore to provide an inexpensive, rust-inhibitive, abrasive material useful for a one-stage, dry, rust-inhibitive air-blasting process for cleaning metal surfaces prior to painting.

Another object is to provide a process for manufacturing such an abrasive material.

BRIEF SUMMARY A typical embodiment of this invention is provided by coating an abrasive such as particles of black slag with a long-chain aliphatic, carboxylic acid to form a rustinhibitive, dry, air-blasting material. The acid forms 1 to 10% of the original mixture, preferably 5%. The mixture is heated to a temperature somewhat above its melting point and then cooled to room temperature while being continuously stirred.

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DETAILED DESCRIPTION To make the abrasive material, a long-chain, aliphatic, carboxylic acid, such as stearic acid, palmitic acid, myristic acid, etc., is mixed thoroughly at room temperature with an abrasive material such as 40 mesh black slag. The mixture is heated to 60 centigrade which is the melting point of stearic acid and then cooled to room temperature, being continuously stirred while cooling.

This process coats the abrasive particles with the acid, preventing the formation of acid dust which results when conventional mixing techniques are employed. Stirring while cooling prevents the agglomeration of particles. The end product is thus a dry, free-flowing material.

Stearic acid is the preferred coating material and a typical coating thickness is 0.2 mil.

The acid may form 1 to 10% of the original mixture by weight and the preferred amount is 5%. It is found that the greater the amount of acid, the greater the rust inhibiting properties of the final material. However, the greater the amount of acid, the greater is the resulting agglomeration of particles. The optimum percentage thus turns out to be about 5%.

The superiority of the stearic acid mixture relative to other Widely used abrasive materials is shown by the results of an actual test. Abrasive blasting was accomplished by means of a Branford Model 1900 Blast Cabinet operated at an air pressure of p.s.i. Abrasive was black slag, 40 mesh. Inhibiting agents examined were zinc granules (20-40 mesh), aluminum granules (20 mesh), and stearic acid crystals. Test specimens were hot-rolled, mild steel, 1 inch by 3 inches by inch thick. Corrosion tests were conducted in an atmospheric corrosion chamber.

The steel specimens were blasted to white metal with the abrasive, to which zinc granules, aluminum granules, or stearic acid was added as rust inhibitors. Control specimens were similarly abrasive blasted, but with no inhibitor present. The zinc and aluminum granules were mixed with the abrasive by stirring. A two-stage process was accomplished by abrasive blasting the steel specimens with no inhibitor, then blasting with the zinc granules without abrasive.

Stearic acid was incorporated into the abrasive first by mixing at room temperature, then heating to 60 C. and cooling to room temperature, continuously stirring the mixture while cooling. This process coats the abrasive particles with stearic acid, preventing formation of stearic acid dust, produced as a result of conventional mixing. Stirring while cooling prevents agglomeration of particles. After blasting, the specimens were exposed for 20 hours in a warm F.), humid, sulfur dioxide-enriched atmosphere. Rust gain was determined by weight diiference. Percent inhibition was calculated as follows:

where I per-cent inhibition, T=average weight of rust on the test coupon (triplicate), and C=average weight of rust on the control coupon (triplicate).

A modification of the atmospheric corrosion process, intended to represent effects of rain, was the following: Abrasive blasted specimens were weighed and totally immersed in stirred, aerated tap water for 20 hours at room temperature. Specimens were removed and the rust formed (principally non-adherent) Was carefully wiped away without disturbing the inhibitive film. Specimens were dried, weighed, and percent inhibition was determined from differences in weight loss. The same specimens were then placed into the atmospheric corrosion chamber for 20 hours, and the percent inhibition was determined as described in the above paragraph.

Degree of rustinhibition is shown in Table l. Thebest inhibition was provided by the abrasive containing percent stearic acid. This conoentration, however, resulted in agglomeration of abrasive particles causing occasional jamming of the blast gun. No adverse effects of this nature were found with stearic acid concentrations of 5 percent and below. The abrasive containing 5 percent stearic acid provided excellent inhibition (even when re-used) in the corrosive atmosphere, with no rust spots visible, and good inhibition under the more drastic conditions of water immersion followed by atmospheric corrosion. The two-stage zinc blasting pl'OCess provided fair inhibition, in contrast to the claims of good inhibition found in the literature, and was almost entirely ineffective with water immersion. In the latter case, the high water resistance considerably reduced the intensity of galvanic protective currents. With the atmospheric corrosion test, the comparatively 'dry metal surface conditions retarded galvanic action of the embedded zinc granules. The stearic acid apparently covered the steel surface more effectively than the metal granules, probably because of surface smearing. The one-stage process with 5 percent zinc in the abrasive was inferior to the two-stage process, indicating a higher surface concentration of zinc On the steel specimens with the latter process. Aluminum granules provided the poorest inhibition, which may be ascribed to the presence of aluminum oxide on the granule surfaces. Zinc or aluminum in powder or dust form was not examined because of the danger of explosion hazard under service conditions. This also applies to magnesium granules and powder.

Film thickness produced by the 5 percent stearic acid was 0.2 mil. This can be readily removed prior to paint application. The film can easily be removed by organic solvent or light abrasive blasting. The resulting surface did not adversely affect paint adhesion. It further stated that any stearic acid remaining would dissolve in the usual paint vehicles and not significantly affect adhesion of Navy vinyl or alkyd coatings to base metal. The presence of up to 5 percent stearic acid was found not to retard rate of scale removal.

TABLE L-EFFECTIVENESS OF INHIBITING AGENTS 1 Inhibited abrasive re-used for third time.

It will be understood that various changes in the details, materials, and arrangements of parts (and steps), which .--have been herein described and'illustrated in order to exmixing particles selected from the group consisting of black slag and metal shot abrasive material solely with a long-chain, aliphatic, crystalline carboxylic acid, said acid comprising 110% by weight of the mixture;

heating said mixture to a temperature which melts the acid;and

cooling said mixture to room temperature while continuously stirring it to prevent agglomeration.

2. A method as in claim 1, wherein said abrasive material consists of particles of black slag.

3. A method as in claim 1, wherein said acid is stearic acid and said temperature is at least C.

4. A method as in claim 1, wherein said abrasive material consists of particles of black slag and said acid is stearic acid comprising approximately 5% by weight of the original mixture.

5. A rust-inhibitive, dry, free-flowing, granular abrasive material for use in the cleaning of metal surfaces by dryair-blasting comprising:

particles selected from the group consisting of black slag and metal shot abrasive material suitable for dry-air-blasting; and

a long-chain, aliphatic, crystalline, carboxylic acid coating on said abrasive particles, 7 said acid forming 110% by weight of the coated particles.

6. A material as in claim 5, wherein said abrasive comprises black slag and said acid comprises stearic acid.

7. A material as in claim 5, wherein said acid is approximately 5% and said abrasive approximately by weight of the original mixture.

References Cited UNITED STATES PATENTS Re. 21,432 4/1940 Lupo 51306 2,318,578 5/1943 Balz et al. 51--304 2,482,667 9/1949 Gray 51-304 2,545,291 3 1951 Lupo 51-304 2,570,904 10/1951 Young 5 l-304 3,020,140 2/1962 Bluth 51-307 3,155,466 11/1964 Grutter 51-307 DONALD J. ARNOLD, Primary Examiner US. Cl. X.R. 51-30 4, 307, 309 

